Set ERR_INVALID_OBEJCT if call failed to server

Tell the client that the last operation failed, instead of continuing doing
useless things.

base-nova/src/base/ipc/ipc.cc

@@ -144,10 +144,11 @@ void Ipc_client::_call()
 
 	/* establish the mapping via a portal traversal */
 	uint8_t res = Nova::call(Ipc_ostream::_dst.local_name());
-	if (res) {
+	if (res != Nova::NOVA_OK) {
 		/* If an error occurred, reset word&item count (not done by kernel). */
 		utcb->set_msg_word(0);
-		PERR("call returned %u", res);
+		/* set return value for ipc_generic part if call failed */
+		ret(ERR_INVALID_OBJECT);
 	}
 
 	_rcv_msg->post_ipc(utcb);

base/include/base/ipc_generic.h

@@ -421,6 +421,14 @@ namespace Genode {
 			 */
 			void _call();
 
+			/**
+			 * Set return value if call to server failed
+			 */
+			void ret(int retval)
+			{
+				*reinterpret_cast<int *>(&_rcvbuf[sizeof(umword_t)]) = retval;
+			}
+
 		public:
 
 			/**